1. Field of the Invention
The invention is directed generally to a method and apparatus that detects wireless transmission problems and, more particularly, to detecting carrier leakage. Moreover, the invention is directed to a method and device to receive and/or recover a transmission frame in the presence of carrier leakage.
2. Related Art
In order to transmit data in a transmission frame in a wireless system from a transmitter to a receiver, including a wireless network, the transmitter must first turn on or power up a power amplifier and/or other related components. Normally the power up process takes about two to three micro-seconds for a power amplifier, and its associated circuitry, to fully power up in order to transmit a transmission frame. During the period of time that the power amplifier is powering up, no transmission of a transmission frame with data takes place. This is done in order to minimize any signal distortion caused by powering up the amplifier. However, the wireless system transmitter may transmit during the power up time period an unmodulated radio frequency carrier prior to the transmission of the transmission frame. The power of this unmodulated radio frequency carrier or carrier leakage in the transmission frame is typically about 20 to 30 dB below the actual signal power of the remaining part of the transmission frame. Moreover, the carrier leakage may have an almost near DC characteristic in that the signal is mostly ones or zeros.
FIG. 1 shows an exemplary transmission frame 100 having carrier leakage 110 in the front portion of the transmission frame 100. In particular, the desired portion of the transmission frame 100 that is to be received in a receiver is the preamble 120, header/signal field 140, and data 160 (payload). It is not desired for a receiver to receive any carrier leakage 110. In this regard, the undesired reception of the carrier leakage 110 is generally minimal when there is a large distance between the transmitter and the receiver. Accordingly, desired signal strength can be still be above the receiver sensitivity level whereas the carrier leakage 110 is then buried in thermal noise and the carrier leakage 110 has little or no effect. Moreover, the reduced power of the carrier leakage 110 (specified at about 20-30 dB below the actual signal power) allows the receiver to be able to avoid receiving this undesired signal in the background.
Carrier leakage is more problematic when the transmitter and receiver are relatively close, such as in current Wireless Local Area Networks (WLAN) systems, for example those compliant with IEEE 802.11, 802.11(a), 802.11(b), 802.11(g), 802.11(n), 802.16, and 802.20, which are being used with increasing frequency in relatively close quarters in homes, businesses, and commercial applications. When the transmitter and the receiver are relatively close, the receiver has a tendency to falsely start receiver processing in response to the carrier leakage 110. In particular, the nearer the receiver to the transmitter and the shorter duration of the energy at an antenna can potentially or falsely start receiver processing, gain control, signal detection, and/or synchronization mechanisms and the like. For example, in some WLAN systems, such as IEEE 802.11(a), the initial symbol timing synchronization relies on certain periodicity of a short preamble. Since the unmodulated radio frequency carrier, including the carrier leakage 110, is near DC at a base band, this fulfills the periodicity requirement and subsequently causes the receiver to faultily trigger and start detection of the transmission frame 100 together with the carrier leakage 110. Since the carrier leakage 110 is much lower in power compared to the actual transmission frame 100 parts including the preamble 120 of the signal, the header/signal field 140, and data 160, when the gain control locks on to the carrier leakage 110, the remaining part of the transmission frame 100 including parts 120, 140, 160 may be saturated in the transmitter and lost once received in receiver. This would subsequently cause a significant degradation in the throughput performance that has been both observed in real operation and environments in the lab, as well as in simulations.